1. Field of the Invention
The present invention relates to a semiconductor device having a compound semiconductor layer composed of BN (boron nitride), GaN (gallium nitride), AlN (aluminum nitride) or InN (indium nitride) or a group III-V nitride compound semiconductor (hereinafter referred to as a nitride based semiconductor) which is their mixed crystal and a method of fabricating the same, and a method of forming a nitride based semiconductor layer.
2. Description of the Background Art
In recent years, a GaN based light emitting semiconductor device (a light emitting semiconductor device based on GaN) has been put to practical use as a light emitting semiconductor device such as a light emitting diode which emits light in blue or violet or a semiconductor laser device. In fabricating the GaN based light emitting semiconductor device, there exists no substrate composed of GaN. Therefore, each layer is epitaxially grown on an insulating substrate composed of sapphire (Al.sub.2 O.sub.3) or the like.
FIG. 18 is a cross-sectional view showing the structure of a conventional GaN based light emitting diode. A light emitting diode shown in FIG. 18 is disclosed in Nikkei Micro Device, February, 1994, pp. 92 to 93.
In FIG. 18, a GaN buffer layer 62, an n-GaN layer 63, an n-AlGaN cladding layer 64, an InGaN active layer 65, a p-AlGaN cladding layer 66, and a p-GaN layer 67 are formed in this order on a sapphire substrate 61. A partial region from the p-GaN layer 67 to the n-GaN layer 63 is removed by etching. A p electrode 68 is formed on the top surface of the p-GaN layer 67, and an n electrode 69 is formed on the exposed top surface of the n-GaN layer 63. Such a structure of the light emitting diode is referred to as a lateral structure.
The light emitting diode shown in FIG. 18 has a pn junction having a double hetero structure in which the InGaN active layer 65 is interposed between the n-AlGaN cladding layer 64 and the p-AlGaN cladding layer 66, and can emit light in blue.
In a conventional GaN based light emitting semiconductor device as shown in FIG. 18, however, dislocations of around 10.sup.9 /cm.sup.2 generally exist in a GaN based semiconductor crystal which is grown on a sapphire substrate depending on the difference in lattice constant between GaN and the sapphire substrate. Such dislocations are propagated from the surface of the sapphire substrate to a GaN based semiconductor layer. In the light emitting semiconductor device composed of the GaN based semiconductor layer on the sapphire substrate, device characteristics and reliability are degraded due to the dislocations.
As a method of solving the problem of the degradation of the device characteristics and the reliability due to the dislocations, epitaxial lateral overgrowth has been proposed. The epitaxial lateral overgrowth is reported in "Proceedings of The Second International Conference on Nitride Semiconductors", Oct. 27-31, 1997, Tokushima, Japan, pp.444-446. FIG. 19 is a schematic sectional view showing the steps for explaining the conventional epitaxial lateral overgrowth.
As shown in FIG. 19(a), an AlGaN buffer layer 82 is grown on a sapphire substrate 81, and a GaN layer 83 is formed on the AlGaN buffer layer 82. Dislocations 91 extending in the vertical direction exist in the GaN layer 83. Striped SiO.sub.2 films 90 are formed on the GaN layer 83.
As shown in FIG. 19(b), a GaN layer 84 is regrown on the GaN layer 83 exposed between the striped SiO.sub.2 films 90. Also in this case, the dislocations 91 extend in the vertical direction in the regrown GaN layer 84.
As shown in FIG. 19(c), when the GaN layer 84 is further grown, the GaN layer 84 is also grown in the lateral direction. Accordingly, the GaN layer 84 is also formed on the SiO.sub.2 films 90. No dislocations exist in the GaN layer 84 on the SiO.sub.2 films 90.
As shown in FIG. 19(d), when the GaN layer 84 is further grown, the GaN film 84 is formed on the SiO.sub.2 films 90 and on the GaN layer 83 between the SiO.sub.2 films 90.
When the epitaxial lateral overgrowth is used, a GaN crystal of high quality having no dislocations can be formed on the SiO.sub.2 films 90.
In a region where the SiO.sub.2 films 90 do not exist, however, the dislocations 91 from the underlying GaN layer 83 extend to the surface of the regrown GaN layer 84. Accordingly, the dislocations still exist on the surface of the GaN layer 84. When the light emitting semiconductor device is fabricated, therefore, a light emitting region must be limited to a region on the SiO.sub.2 films. Therefore, it is impossible to increase the size of the light emitting region.
When the area of the SiO.sub.2 film is increased in order to increase the area of the GaN layer of high quality, the surface of the GaN layer which is grown in the lateral direction cannot be flattened.
In the conventional GaN based light emitting diode shown in FIG. 18, the sapphire substrate 61 is an insulating substrate. Therefore, the n electrode 69 cannot be provided on the reverse surface of the sapphire substrate 61, and must be provided on the exposed surface of the n-GaN layer 63. Therefore, a current path between the p electrode 68 and the n electrode 69 is longer, so that an operation voltage is higher, as compared with those in a case where the n electrode is provided on the reverse surface of a conductive substrate.
Furthermore, when a GaN based semiconductor laser device is fabricated, it is difficult to form cavity facets by a cleavage method as in a semiconductor laser device for emitting red light or infrared light using a GaAs substrate.
FIG. 20 is a diagram showing the relationship between the crystal orientations of a sapphire substrate and a GaN based semiconductor layer. In FIG. 20, an arrow by a solid line indicates the crystal orientation of the sapphire substrate, and an arrow by a broken line indicates the crystal orientation of the GaN based semiconductor layer.
As shown in FIG. 20, the a-axis and the b-axis of the GaN based semiconductor layer formed on the sapphire substrate are shifted 30.degree. away from the a-axis and the baxis of the sapphire substrate.
FIG. 21 is a schematic perspective view of a semiconductor laser device composed of a GaN based semiconductor layer formed on a sapphire substrate.
In FIG. 21, a GaN based semiconductor layer 70 is formed on a (0001) plane of a sapphire substrate 61. A striped current injection region 71 is parallel to a &lt;1120&gt; direction of the GaN based semiconductor layer 70. In this case, a {1 100} plane of the GaN based semiconductor layer 70 is inclined at 30.degree. to a {1100} plane of the sapphire substrate 61. Both the sapphire substrate 61 and the GaN based semiconductor layer 70 are easily cleaved along the {1100} plane.
The respective cleavage directions of the sapphire substrate 61 and the GaN based semiconductor layer 70 thus deviate. When a GaN based semiconductor laser device is fabricated, therefore, it is difficult to form cavity facets by a cleavage method, as in the semiconductor laser device for emitting red light or infrared light which is formed on the GaAs substrate. In this case, the necessity of forming the cavity facets by etching is brought about. When the cavity facets are formed by etching, however, it is impossible to reduce an operation current of the semiconductor laser device because it is difficult to form facets perpendicular to the substrate.
On the other hand, various reports and proposals are made with respect to methods of controlling a transverse mode of the GaN based semiconductor laser device. Almost all of the methods of controlling the transverse mode include two types, i.e., a ridge waveguided structure and a self-aligned structure which are employed by the conventional semiconductor laser device for emitting red light or infrared light.
Since the GaN based semiconductor layer is chemically stable, however, it cannot be patterned by wet etching, unlike an AlGaAs based semiconductor layer used for the conventional semiconductor laser device emitting red light or infrared light, and must be patterned by dry etching such as RIE (Reactive Ion Etching) or RIBE (Reactive Ion Beam Etching).
In the GaN based semiconductor laser device, therefore, the patterning for fabricating the ridge waveguided structure or the self-aligned structure cannot be performed easily and with good reproducibility. Moreover, device characteristics greatly vary depending on the precision of the dry etching.